Liz, I scoured the internet for a couple hours this morning, trying to nail that down. Saw a lot of fascinating ideas and concepts, but there is a real shortage of definitive information on human autophagy. A mouse will be dead in 2 or 3 days if it doesn’t eat; obviously humans are massively different. Studies are costly, and I guess there isn’t much money in people eating and buying less.
One thing that strikes me today is that some important benefits of fasting - and things attributed to autophagy - occur upon starting to eat again, or “refeeding.” There obviously has to be a period of fasting to set this up - and perhaps it’s some small number of days. I’d think there will be individual variation here, in how responsive a person is to the process, how much glycogen, etc., is stored in their liver and other body parts, and so forth. The good news is that it doesn’t matter too much that we cannot pinpoint one’s “peak” for autophagy, short of intensive testing of blood and tissue, as there are benefits regardless of the exact duration, just as there are health benefits for most of us in missing even just one meal, or eating one meal per day.
There are different phases and functions with autophagy - I picture it like multiple sine waves - and they won’t all have their peak at the same time. The extra and damaged molecules (primarily proteins) within a cell start to be cleaned up first, then damaged or unneeded larger parts of the cell itself, and still later liquid droplets within the cell. These phases overlap, and the insulin/glucagon concentrations affect the timing of them, thus some individual variation will be present there too. For many of us, years of eating polyunsaturated vegetable oils have clogged cells with droplets of them, messing up the way the cell works. That working on the liquid droplets is the last phase of autophagy to begin, argues fairly strongly for longer fasts rather than shorter, in my opinion.
The benefits of fasting and autophagy are very cumulative - repeated cycles of fasting and refeeding give more and deeper changes for good than does one cycle, regardless of the length of that cycle.
For human growth hormone secretion, free fatty acids in the blood tend to suppress HGH secretion. This is already a long post, and I’m trying not to load it up with links, but I quote from the one study I will link to: “Free fatty acid lack itself seems to be the signal for HGH release…” In the study, lowering free fatty acid levels resulted in an increase in HGH, with a lag time of two hours.